Transcript Auto-titrating pH Meter

Auto-titrating pH Meter
PETER BAKER
CAROLE CHEN
MICHAEL HERNANDEZ
FALL 2009
MECHATRONICS
PROF. KAPILA
http://www.techneusa.com/ph/3510.jpg
Outline
 Objective
 Approach
 Mechanical design
 Electrical components
 Flow chart of our program
 Results
 Cost Estimate
 Conclusion
 Failure/success
 Suggestions
Objective
 What is pH?


pH = -log [H+]
How is this important?
 Design a cost-effective pH meter (using BS2 as our
microcontroller)
 Continuously check pH of solution
 Be able to create solution of desired pH

based on user’s input
Approach : Mechanical Design
 Initial Design
 Final Design
Circuit : Electrical Components
 Materials needed :
10K Potentiometer
TL082 Dual BiFET OP Amp
ADC0831 A/D converter
Three continuous servo motors
pH probe sensor
9V snap connectors
Various resistors
Various jump wires
3 Normally Open Push-button switches
Electrical Basis of Project
 Measuring small voltages = .060 V per pH unit change
 Our pH range : pH 1- pH 7
 Neutral pH 7 : 0.0 V
As we move down pH : pH 7  pH 6, increase in voltage by 0.060 V
e.g. At pH 4, 3 units from neutral pH
0.060 V/pH unit x 3 pH units from neutral pH = .180 V or 180 mV reading
pH Probe
pH probe model number 03847K : $ 60
High source impedance : glass membrane
Voltages cannot simply be measured with a DMM
In addition, voltages are very low (0-0.360V)
How to interface with BS2?
Operational Amplifier
Originally used an LM358 op-amp included in the BS2 kit
However, all op-amps are not ideal :
Golden Law of op-amp circuitry : input current into
terminals should be zero. In reality, this is not the case
Small current converted into small voltage and also gets
amplified!!
Results in output error in calculations
LM358 op-amp did not work with our setup
TL082
 High input impedance, available at local RadioShack : $2
 Powered using 2 9V batteries : $10
 Results using TL082 : Priceless
Non-Inverting Amplifier Circuit
Vout = Vin (1+Rf/Ri)
Gain : Amplification Factor
(1+Rf/ Ri )
Rf = 10 kΩ
Ri = 1 kΩ
Gain = 11
Amplified Signal
.035 V- 4.03
Digitization
Vin (+) : input analog signal from pH probe needed to be
digitized
Vin (-) : offset , 0V
Vref : Set to 4V
Span = 4.03 V - .035 V = about 4.0 V
Quantization : (4.03 V - .035)/256 = ~ 16 mV per step
After ampification each pH unit 60 * 11 = 660 mv per pH
unit ~ 42 steps per unit pH change
Circuit Diagram
Program Flow Chart:
Cost Estimate
Materials
Estimated Cost
10K Potentiometer
*
ADC0831 A/D convertor
*
Three continuous servo motors
*
Various resistors
*
Various jump wires
3 Normally Open Push-button
switches
*
*
BS2 kit
$200.00
TL082 Dual BiFET OP Amp x 3
$6.00
pH probe sensor
$60.00
9V snap connectors
$3.00
Ring clamps x 2
$20.00
9V Battery x 2
$10.00
Tools/ misc
$20.00
* = included in BS2 kit
Total Cost
$319.00
Problems Encountered
 Initial design failures/flaws
 Leakage
 Stability
 BS2’s EEPROM
 Used maximum amount of space available

Not able to incorporate programming codes for keeping pH within desired
range (i.e. not only go to desired pH value)
 Fluctuations of probe readings
 Need to use the shortest lead possible
 Limited pH range (i.e. 4.0 to 7.0) ???
 Pro: better resolution
 Con: inability to detect basic pH (from 8.0 to 14.0)
Results (to be shown in demonstration)
Buffer
Salt
pH
mV Start
mV 3 min
50 mM NaPho
500 mM
7.0
49 mV
63 mV
50 mM NaPho
0 mM
7.0
37 mV
53 mV
50 mM Tris
500 mM
7.0
5 mV
10 mV
50 mM Tris
0 mM
7.0
18 mV
55 mV
100 mM NaPho
500 mM
7.0
48 mV
47 mV
100 mM NaPho
0 mM
7.0
46 mV
43 mV
Thank you!
Prof. Kapila, Chandresh, Alex,
& all of our classmates for your valuable advice
Now … *drum rolls*
DEMONSTRATION TIME
*cross your fingers*